Toner concentration detector

ABSTRACT

An improved apparatus for applying toner solution to a charged recording medium wherein the concentration of toner particles in the solution is continuously monitored and adjusted is disclosed. The apparatus includes means to measure toner concentration comprising a light source, a transparent chamber receiving a continuously flowing stream of toner solution and means for measuring light intensity from the light source as passed through the transparent chamber. Preferably the means for measuring light is particularly sensitive to light in the near infrared region (about 8500 Angstroms). The output signal from the means for measuring light is a measure of the concentration of toner particles in the toner solution and is used to add toner concentrate to the toner solution as the concentration of toner particles in the solution is depleted. In preferred embodiments, means for controlling the output from the light source and for measuring toner concentration independent of the position of the light source are disclosed.

United States Patent Muth 5] Dec. 16, 1975 TONER CONCENTRATION DETECTORPrimary Examiner-Mervin Stein Assistant Examiner-Steven Hawkins [75]Inventor. 315:? M. Muth Oklahoma Clty Attorney, Agent, or FirmRonald T.Reiling [73] Assignee: Honeywell Information Systems Inc., 57 ABSTRACTWaltham, Mass. An improved apparatus for applying toner solution to [22]Filed: Mar. 4, 1974 a charged recording medium wherein the concentrationof toner particles in the solution is continuously [21] Appl' 44770lmonitored and adjusted is disclosed. The apparatus includes means tomeasure toner concentration com- [52] U.S. Cl 118/7; 1 18/637 prising alight source, a transparent chamber receiving [51] Int. Cl. B05B 5/02 acontinuously flowing stream of toner solution and [58] Field of Search118/637, DIG. 23, 7, 9, means for measuring light intensity from thelight 118/10, 11; 117/ 17.5 source as passed through the transparentchamber.

Preferably the means for measuring light is particu- [56] ReferencesCited larly sensitive to light in the near infrared region UNITED STATESPATENTS (about 8500 Angstroms). The output signal from the 3,430,6063/1969 Pease et a1. 118/7 means for measuring light is a measure of h3,494,328 2/1970 Maloney H 118/637 tration of toner particles in thetoner solution and 15 3,635,373 1/1972 Kuhl et a1? H 222/57 used to addtoner concentrate to the toner solution as 3,712,203 l/l973 Kishi et all A 7. 95/89 R the concentration of toner particles in the solution is3,727,065 4/1973 Maksymiak 250/218 depleted. In preferred embodiments,means for con- 3,752,119 8/l 73 Matkan 1 1 11 trolling the output fromthe light source and for mea- 3-756v192 9/1973 Locklar et 4 1 18/7suring toner concentration independent of the position 3,777,173 12/1973Landrith 118/637 of the light Source are disclosed 3,819,948 6/1974Lljlma et a1 250/559 8 Claims, 6 Drawing Figures law US. Patent Dec.16,1975 Sheet20f4 3,926,145

U.S. Patent Dec. 16, 1975 Sheet3of4 3,926,145

U.S. Patent Dec.16, 1975 Sheet4of4 3,926,145

TONER CONCENTRATION DETECTOR BACKGROUND OF THE INVENTION This inventionpertains to an improved apparatus for printing upon a recording mediumand particularly, to printing permanent images on a charged paper, suchas an electrographic paper, at high speeds.

High speed, electrographic printing is used to pro duce a computerprintout by non-impact printing tech niques. Generally, electrographicprinting utilizes an electrographic paper medium composed of aconductively treated base that supports a plastic dielectric coating.Latent images are formed on the paper by positioning the electrographicpaper between an electrode that contacts the conductive base and asecond electrode. The surface of the second electrode conforms to theshapes to be printed or, in an alternative embodiment, the secondelectrode can be selectively activated to form a predetermined image.Thus, this second electrode can have a fixed format and/or a variableformat to produce the shapes to be printed on the dielectric paper. Ahigh voltage (i.e., 500 to 800 volts) applied between the two electrodesexcites the paper medium, thereby establishing an electrostatic fieldacross the dielectric coating. The dielectric coating retains a residualelectrostatic field that constitutes a charged latent image of theshapes to be printed.

Latent images on the charged paper are developed by contacting the paperwith a toning liquid composed of charged carbon-resin particlessuspended in a liquid carrier. The liquid carrier is preferably a highlyparrafinic solvent such as Isopar L which also functions to soften thecharged carbon-resin particles. The residual electrostatic field on thedielectric surface of the charged paper attracts the carbon-resinparticles and holds them in position. This makes the latent imagevisible. The visible image is then fixed, i.e., made permanent, bydrying and applying heat to remove the liquid carrier and to solidifythe carbon-resin particles and bond them to the paper. A suitable methodand apparatus for high speed electrographic printing is described in US.Pat. No. 3,701,337, the teachings of which are incorporated by referenceherein.

In a high speed electrographic printing operation such as illustrated inUS. Pat. No. 3,701,337, the charged printing medium is contacted with arecirculating stream of toner solution. The excess toner solution isrecovered and recycled for future use. However, since toner particlesare removed from the toner solution in developing the latent images, theexcess toner liquid recovered after application of the toner solution tothe excited paper contains a lesser amount of toner particles. As aconsequence, when the toning solution is continuously recycled,additional toner particles must be added to the circulating solution tomaintain a constant, high quality printout.

In the prior art, this was accomplished by the operator visuallydetermining the quality of the printout. When the operator noticed thatthe printout quality was declining and the developed characters were notsharp, he would, at his discretion, manually add additional tonerparticles, typically in the form of a concentrated solution orsuspension, to the circulating toner solution. As a consequence, in themanual prior art method of addition, printout quality frequently varied.Further, in high speed electrographic printing wherein printing speed inexcess of 1,000 lines per second commonly occurs, the operator wouldfrequently have to add toner concentrate to the system. Thus, when themachine operated at a high printout rate with a large amount of printingper page, it was difficult for the operator to maintain a constant printquality because of the frequent need to add toner concentrate to thecirculating toner solution.

SUMMARY OF THE INVENTION It is an object of this present invention toprovide a printing apparatus for developing latent charged images on arecording medium with a toner solution wherein the concentration oftoner particles in the toner solution is continually monitored andcontrolled so as to guarantee the production of uniformly high qualityprints.

It is another object of this invention to provide a printing apparatuswhich will automatically add a con centrated toner solution to a workingtoner solution so as to maintain a relatively constant toner particleconcentration in the working toner solution.

It is another object of this invention to provide a printing apparatusfor producing high quality images wherein light is passed through atoner solution, the resultant light intensity is measured, and thismeasured light intensity is used as a means for determining when to addtoner concentrate to the toner solution.

It is a specific object of this invention to provide a printingapparatus which measures the concentration of toner particles in a tonersolution by measuring the intensity of light passed through the tonersolution wherein means are employed for providing a constant intensitylight source so as to insure an accurate mea surement of the tonerparticle concentration.

Although a typical toner solution is substantially opaque, it has beendiscovered that the intensity of light passed through a thin layer oftoner solution is inversely proportional to the concentration of tonerparticles in the toner solution. In other words, a higher concentrationof toner particles in the toner solution produces a lower lightintensity. This light intensity, I, can be represented by the generalformula wherein K and K are constants depending on the type of toner,the intensity of the light source, and the sensors used to sense thelight intensity; and C is the concentration of toner particles in thetoner solution being measured. This light intensity can be determined bymeasuring the voltage or current emitted by a conventional photoelectricor solar cell. This current or voltage, in turn, can be electronicallyprocessed to provide a measurement of concentration and a command to addtoner concentrate to the circulating toner solution when theconcentration of toner particles falls below a predetermined value.

In a broad embodiment, therefore, the present invention relates to animprovement in an apparatus for applying a working toner solutioncomprising a suspension of black or colored toner particles in avolatile carrier to a charged recording material containing a latentimage in a toning station to render the latent image visible. Theparticular improvement of the present invention includes means forcontinuously measuring the concentration of toner particles in theworking toner solution, transfer means for transferring concentratedtoning solution from a reservoir to tlie wfirking solution, andoperating means for operatiiiitlie transfer means when the tonerconcentration fa ls b elow a predetermined value as determined by themeasuring means and for terminating the operation of the transfer meanswhen the toner concentration returns to the predetermined value.

A preferred apparatus for measuring the toner concentration according tothe broad embodiment specified above includes a light source, atransparent chamber spaced-apart from the light source to receive acontinuously flowing stream of working toner solution, means formeasuring the light intensity from the light source after the lightpasses through the chamber and means for focusing the light from thelight source through the transparent chamber. Preferably, the means formeasuring the light intensity is particularly sensitive in the nearinfrared range; a conventional solar cell having optimum sensitivity inthe range of 8500 Angstroms is particularly preferred. The use of lightin the infrared region produces particularly accurate, reliable andreproducible results when working with solutions of high opacity, suchas a working toner solution. The thickness of the measuring chamber istypically about 0.01 to 0.05 inches thick when dealing with relativelyhigh opacity solutions. However, for different solutions, the width ofthe chamber can be up to 0.25 inches.

In a more limited embodiment, the apparatus of the present inventionfurther includes means for detecting the amount of light passed to thetransparent chamber, means for providing current to the light source andmeans for adjusting the current to the light source in response to theamount of light passed to the chamber to maintain a predeterminedconstant light level.

In another limited embodiment, the apparatus of the present inventionincludes means for detecting the amount of light passed to the chamberfrom the light source and means for comparing the light intensity oneither side of the chamber, as determined by the detection'means, saidcomparison means providing an accurate measurement of tonerconcentration irrespective of the output from the light source.

In another specific alternate embodiment of the present invention, theapparatus is equipped with means for measuring the uninterrupted lightintensity from the light source at a distance from the light sourceequal to the distance between the measuring means positioned behind thechamber and the light source. This distance is measured by following thelight path from the light source to the specified measuring means. Alsoincluded is means for comparing the uninterrupted light inten sity infront of the chamber to the light intensity behind the chamber, therebyproviding an accurate measurement of toner concentration irrespective oflamp placement. This embodiment is particularly useful since thedimensions of lamps and filaments within lamps can vary substantially.In a conventional device in which the length of the light paths from thelight source to the means for measuring are unequal, the substitution ofone lamp for another can vary the ratio of the light intensitiesreceived by the means for measuring. When the two means for measuring,however, are equidistant from the light source, any effect due tovariations of the lamp dimensions is removed. Typically, the means formeasuring uninterrupted light intensity is positioned in front of thechamber and includes means for reflecting of the light from the lightsource to this light intensity measuring means.

Other objects and embodiments and a more detailed description of theforegoing embodiments will be found in the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram ofa nonimpact printer employing a preferred embodiment of the presentinvention which automatically adjusts the concentration of tonerparticles in the working toning solution;

FIG. 2 is a detailed cross-sectional view of a specific device tomeasure the concentration of toner particles in a toning solutionaccording to the present invention;

FIG. 3 is a schematic diagram illustrating the operation of the deviceillustrated in FIG. 2;

FIG. 4 is a schematic diagram illustrating the operation of an alternatedevice for measuring toner concentration in FIG. 1, wherein an accuratemeasurement of toner concentration is obtained irrespective of theposition of the light source; and

FIGS. 5A and 5B are circuit diagrams of the operating circuitschematically illustrated in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2,there is schematically illustrated an apparatus for applying toningsolution to a charged electrographic medium. This apparatus includes astorage tank 2 containing a working toning solution which is passedthrough a line 30, a toner concentration detector 32 and a line 4 totoning station 6. Within detector 32, the bulk of the toning solutionpasses through cavities 35 and 37. Within toning station 6, workingtoning solution is applied to a charged electrographic paper 8containing a latent image to produce a wet paper 10 containing a visibleimage. This paper in turn, is dried by application of heat in dryingstation 14 to produce a dried electrographic paper 16 containing a fixedvisible image. The evaporated carrier is removed from drying station 14via line 18 and recycled back to storage tank 2, or is removed from theprocess via line 20. The toning solution passed to toning station 6 thatis not deposited on electrographic paper 8 is removed from toningstation 6 via line 12 and is returned to storage tank 2 with a depletedtoner particle concentration by any conventional means, such as a pump(not shown).

To maintain a constant predetermined level of toning particles in theworking toning solution, a slip stream from cavities 35 and 37 thatflows through a chamber 64 is continuously monitored. When theconcentration of toner particles in the working solution falls below apredetermined value, concentration detector 32 energizes pump 26 viacontrol line 34 and a concentrated toner solution withdrawn fromconcentrate tank 22 via line 24 is passed to storage tank 2 via line 28.

Pump 26 comprises a plunger P which reciprocates in a cylinder C. Eachtime plunger P-moves to the right as shown in FIG. 1, a charge of tonerconcentrate spurts into tank 2 through line 28. Plunger P is moved tothe right by conducting a current pulse through a conventional solenoidcoil 29. As soon as the current is terminated, plunger P is returned tothe normal position shown in FIG. 1 by resilient means not shown. Byincreasing the repetition rate of the pulses applied to coil 29, therate at which toner concentrate is added to tank 2 is likewiseincreased.

The addition of toner concentrate via pump 26 continues until theconcentration level of toner particles in the working solutioncirculated through line 30 reaches a desired value. In a typicalembodiment, the concentration level which actuates pump 26 is the sameconcentration level at which pump 26 is deactivated. The sameconcentration level or set point can be used for actuating anddeactuating pump 26 because of the large inventory of toning solutioncontained in tank 2 and circulated through a typical non-impact printerapparatus. Actuating pump 26 when the toner concentration level isfalling below the desired value does not immediately result in anincreased concentration level at toner concentration detector 32.Because of the large inventory of toning solution and the physicalplacement of the tank 2 relative to toner concentration detector 32, thetoner concentration level may actually continue to decline at detector32 for a time before it begins to increase due to the toner concentrateadded at tank 2. It will take a period of time for the concentration oftoner particles in line 30 to build up to the desired value after pump26 has been actuated.

Preferably, toning concentration detector 32 in cludes means to increasethe addition rate via pump 26 when the concentration of toning particlesin the working solution falls below a second predetermined value lessthan the desired value. This feature allows the apparatus of the presentinvention to rapidly compensate for any large imbalances in theconcentration of toner particles in the toning solution and to rapidlycorrect the concentration so as to ensure a constant quality printout.

In a typical embodiment, the working toner solution is maintained at 4.0:t 0.5 wt. total concentrate, wherein the concentrate contains 17%solids. In other words, the working solution contains about 0.595 toabout 0.765 wt. solids. However, the amount of solids in the workingsolution is a function of the identity of the specific toner solutionbeing used and the degree of clarity desired on the final developedelectrographic paper.

Referring to FIG. 2, there is illustrated a preferred tonerconcentration detector 32. Toner concentration detector 32 comprises anincandescent light bulb 38 containing a filament 40 positioned in lightbulb socket 36 located within housing 33. The light from light bulb 38is directed through lenses 42 and 44 mounted in lens holder 46, throughglass plates 58 and 60 before impinging on light intensity detector 68.Light intensity detector 48 is positioned immediately in back of forwardlens 44 and measures the light intensity that is passed to glass plate58. Light intensity detectors 48 and 68 are conventional silicon solarcells of the type manufactured by Sensor Technology, Inc. These cellsare short-circuited and the current generated therein is indicative ofthe light intensity falling upon the detec tor. The current generated indetector 48 is passed via lead 50 to standoff 52 and the currentgenerated in detector 68 passed via lead 72 to terminal 70. The functionof these current values in the apparatus of the present invention willbe explained in further detail by reference to FIGS. 3 and 4hereinafter.

The light passing from lens 44 passes through a predetermined distance54 before contacting front glass plate 58. The spacing between glassplate 58 and lens 44 is adjusted by washers 56. Second glass plate 60 ismounted in fixed end piece 66 and is spaced apart from first glass plate58 to define a space or chamber 64 having a predetermined thickness. Inaccordance with the present invention, working toning fluid is removedas a slip stream from cavities 35 and 37 and is passed through chamber64 to measure the concentration of toning solids in the toner solution.Seals 62 provides a tight fit between glass plate 58 and housing 33 andbetween glass plate 60 and end piece 66. O-ring 67 provides a leak prooffit between end piece 66 and housing 33.

In operation, the intensity of the light emitted from light bulb 38 andfocused by lenses 42 and 44 is determined by light intensity detector48. The light then passes through transparent glass plate 58 and theworking fluid contained within chamber 64, thereby reducing theintensity of the light passing through plate 60. The light strikinglight intensity detector 68 is compared with the original lightintensity determined by detector 48 to provide a reliable, accuratemeasurement of the concentration of toning particles in the tonersolution passed to chamber 64.

Referring to FIG. 3, there are illustrated two embodiments foraccurately measuring the concentration of toning particles in the tonersolution. In one embodiment, to ensure that light bulb 38 continues toemit a light of a predetermined constant intensity, the light intensityis continuously monitored by detector 48 that is connected by conductors48A and 48B to an amplifier 78 which forms part of an operating circuit79. The current produced by detector 48 is amplified by ampli fier 78 toprovide a control signal. In one embodiment, this control signal ispassed via circuit 80 to current regulator 74 which controls the inputof current to light bulb 38 from a current source 76. This configurationprovides a constant intensity emission of light from light bulb 38.

In FIG. 3, detector 68 produces a DC current proportional to theintensity of the light that passed through glass plate 58, the tonersolution contained in chamber 64 and glass plate 60. Detector 68 isconnected to an amplifier 82 by conductors 68A and 68B. The resultantcurrent produced by detector 68 is amplified by current amplifier 82 toproduce another control signal on a conductor 84. Conductors 84 and 86are connected to a control circuit 88 which produces a pulsating voltagethat drives plunger P at a predetermined rate when the concentration ofthe toner particles in the toner decreases below a predetermined value.The embodiment illustrated in FIG. 3 provides a reliable and accuratemethod of determining toner concentration that is not sensitive to theefficiency of lamp 38.

A preferred form of operating circuit 79, including amplifiers 78 and82, as well as control circuit 88, is shown in more detail in FIG. 5,consisting of FIGS. 5A and 5B. In FIG. 5, detectors 48 and 68 arerepresented as current generators having positive terminals 92, 93 andnegative terminals 95, 96. In response to light, the detectors generatean electrical current that flows in the direction of arrows I.

Amplifier 78 comprises a type 747 operational amplifier having aninverting input 102, a noninverting input 104 and an output 105. Theoperational amplifier is controlled by resistors 106-108, apotentiometer 110 having a slider arm 111 and a capacitor 114, allconnected as shown.

Amplifier 82 comprises a type 741 operational amplifier 120, having aninverting input 122, a noninverting input 124 and an output 126.Amplifier is controlled by resistors 128432, potentiometers 134 and 135having slider arms 136 and 137, respectively, and capacitors 140143, allconnected as shown.

A power supply (not shown) furnishes plus 12 volts DC over conductors144-147 and minus 12 volts DC over conductors 150-155. A plus volt DCsignal is transmitted by the power supply to conductors 158 and 159.

Control circuit 88 comprises a bipolar transistor 166 having a base 168,an emitter 169 and a collector 170. Circuit 88 also includes anotherbipolar transistor 172 having a base 174, an emitter 175 and a collector176, as well as a unijunction transistor 180 having a base two 182, abase one 183, and an emitter 184. Circuit 88 is controlled by diodes188-192, resistors 196-208 and a capacitor 210. A NAND gate 211 suppliespulses to a one shot multivibrator 212 that, in turn, supplies currentpulses through conductor 34 to solenoid coil 29. A pump inhibitconductor 215 may be switched to a logical 0 state to prevent theoperation of pump 26.

Control circuit 88 is functionally divided into analog circuits 213,214; switching circuits 215, 216; and a signal generator 218:

Analog circuit 213 comprises resistors 196, 197 which algebraically addand scale the amplified signals on output conductors 84 and 86 toproduce a switching signal that is transmitted to the base of transistor172. Analog circuit 214 comprises resistors 198, 199 which algebraicallyadd and scale the amplified signals on output conductors 84 and 86 toproduce another switching signal that is transmitted to the base oftransistor 166. Since algebraic addition also includes subtraction,circuits 213, 214 can accommodate both neg ative and positive voltageson conductors 84 and 86. Since transistors 166 and 172 are switched fromtheir conductive to their nonconductive states by the application ofvoltages to their bases near ground potential, the ratio of the voltageon conductor 86 to the voltage on conductor 84 at which transistor 166and transistor 172 switch state is nearly constant irrespective ofchanges in the intensity of lamp 38, so that the detection of tonerconcentration is independent of lamp intensity.

Switching circuit 215 disables signal generator 218 when theconcentration of toner particles becomes too great, and switchingcircuit 216 quadruples the repetition rate of singal generator 218 whenthe concentration of toner particles becomes too dilute.

A lamp indicator circuit 220 may be used in order to monitor thecondition of light bulb 38. The circuit comprises a type 747 operationalamplifier 222 having an inverting input 224, a noninverting input 226and an output 228. Output 228 drives a bulb 231 through an amplifier 229having its input voltage controlled by a 4.7 volt Zener diode 230. Theoperational amplifier is also controlled by resistors 232-237 and bycapacitors 240-242.

A dilute toner indicator circuit 250 may be used in order to indicate toan operator the condition of the toner concentration. The circuitcomprises a flip-flop 252 consisting of NAND gates 254 and 255. Thecircuit also includes another NAND gate 258, as well as diodes 260, 261,resistors 264, 265 and capacitors 268, 269.

An output conductor 270 is connected through an' amplifier 273 to anindicator bulb 271.

In order to adjust amplifier 78, lamp 38 is turned on the slider arm 111is moved until amplifier 100 produces a plus 8 volt DC signal onconductor 86. In order to achieve this result, detector 48 shouldproduce 0.16 to 0.01 milliamps of current. In order to adjust amplifier82, tank 2 is loaded with a toner solution of normal concentration andpotentiometer slider arm 136 is moved until amplifier produces a 2 voltDC signal on conductor 84. The adjustment of slider arm 136 willaccommodate types of detector 68 capable of producing between 8.0 and0.4 microamps of current. If detector 68 produces less than 0.1microamps of current, slider arm 137 also may need to be adjusted.

Because of the reverse polarity connection of detectors 48 and 68 toamplifiers 78 and 82, the amplifiers produce amplified signals onconductors 84 and 86 which represent the detector current signalsinverted with respect to each other.

When amplifiers 78 and 82 are adjusted to produce the normal conditionson conductors 84 and 86 described above, the base of transistor 172 isbiased near ground potential so that the transistor is switched to itsnonconductive state (i.e., turned off). However, the base of transistor166 is biased slightly above ground potential so that the transistor isturned on and driven into saturation. As a result, diode 189 is forwardbiased and diode 190 is reversed biased so that capacitor 210 is chargedfrom 12 volt supply conductor 146 through a 270 K resistor 205. Thevalues of resistor 205 and capacitor 210 are arranged so that capacitor210 charges to the emitter firing voltage of unijunction every 2seconds. When unijunction 180 fires, current is drawn through emitter184 and resistors 207, 208, so that a positive pulse of voltage istransmitted to NAND gate 211, and multivibrator 212 produces a currentpulse having a predetermined duration. In this mode of operation,unijunction 180 operates as a pulse oscillator or generator having aperiod of 2 seconds. As a result, once every 2 seconds, plunger P ofpump 26 (FIG. 1) pumps a charge of toner concentrate into tank 2,thereby increasing the concentration of toner particles in the workingtoner solution. If the concentration of the toner particles in tank 2increases slightly above the normal concentration, the voltage onconductor 84 increases slightly above minus 2 volts DC, therebyswitching transistor 172 to its conductive state. At this point in time,diode 192 is forward biased so that emitter 184 of unijunction 180 isheld to a low voltage, thereby preventing the unijunction from firing.In this mode of operation, no signals are transmitted to pump 26 and theconcentration of toner particles cannot increase. At this same time,diode 261 is forward biased, thereby transmitting a logical 0 signal toflip-flop 250 over conductor 272. This signal forces NAND gate 255 toproduce a logical 1 signal of plus 5 volts on conductor 270. The plus 5volt signal causes amplifier 273 to light bulb 271, thereby indicatingto an operator that the toner particle concentration is within anappropriate range of values.

As toner solution is applied to recording medium 8 in toner station 6,the toner particle concentration in tank 2 decreases, and the voltage onconductor 86 also decreases to minus 2 volts DC. At this point in time,transistor 172 again is switched to its nonconductive state and diode192 is reverse biased, thereby enabling the production of pulses havinga period of 2 seconds from unijunction 180. At the same time, diode 261is reverse biased so that a logical 1 signal is transmitted to conductor272. However, a logical 1 signal continues to be produced on conductor270 and bulb 271 continues to glow, so that an operator will know thatthe toner particle concentration is adequate.

If the concentration of toner particles in tank 2 continues to decreasebelow the normal concentration in spite of the operation of generator218 at 2 second intervals, the voltage on conductor 84 continues todecrease. When the concentration decreases to a too dilute value atwhich the voltage on conductor 84 is about minus 7 volts DC, transistor166 is switched to its nonconductive state (i.e., turned off). At thispoint in time, diode 189 is reverse biased and diode 190 is forwardbiased to place resistor 204 (having a value of 47 K ohms) in parallelwith resistor 205. This mode of operation decreases the time constant ofthe charging circuit for emitter 184 so that unijunction 180 firesapproximately every one-half second. As a result, pump 26 operates 4times as fast as the normal rate. That is, the plunger operates everyhalf second instead of every 2 seconds.

When transistor 166 is turned off, diode 260 is reverse biased, therebytransmitting a logical 1 signal to NAND gate 258. NAND gate 258, inturn, transmits a logical signal over conductor 274 which causesflipflop 252 to change state, thereby switching conductor 270 to groundpotential and turning off bulb 271.

After the 2 stroke per second pumping rate of pump 26 increases thetoner particle concentration so that the voltage on conductor 84increases above minus 7 volts DC, transistor 166 is turned on, therebydecreasing the pump rate to one stroke in 2 seconds. At this point intime, diode 260 is forward biased so that a logical 1 signal istransmitted to conductor 274. However, flipflop 252 continues to holdconductor 270 at ground potential and bulb 271 remains off. Bulb 271remains off until the toner particle concentration increases slightlyabove the normal concentration, at which time transistor 172 is turnedon in the manner previously described. Bulb 271 exhibits hysteresis dueto the operation of flipflop 252 which provides a reliable indication oftoner particle concentration.

Lamp indicator circuit 220 is adjusted so that output 228 is at groundpotential and bulb 231 is turned off when lamp 38 has at least averageintensity. However, if the intensity of lamp 38 decreases below apredetermined value, the voltage on conductor 86 will decrease. When thevoltage reaches about 4.7 volts, amplifier 222 produces a plus 4.7 voltsignal onoutput 228. The signal causes amplifier 229 to turn on lamp231, thereby indicating that lamp 38 requires replacement. Resistors 234and 235 provide about 0.21 volts of the hysteresis to prevent flickeringof lamp 231 when conductor 86 slowly changes voltage near the plus 4.7volt level.

The embodiment illustrated in FIG. 3 is sensitive to the position offilament 40 relative to detectors 48 and 68. As can be seen in FIG. 3,detectors 48 and 68 are each positioned at different distances fromfilament 40. Although the socket for light bulb 38 is fixed, theposition of the filament within the light bulb can vary. If suchvariance regularly occurs, detectors 48 and 68 would have to berecalibrated each time light bulb 38 is replaced. By use of theapparatus illustrated in FIG. 4, these deficiencies are avoided. Inparticular, detector 48 is positioned at the same distance from filament40 as detector 68 as measured along the light path from filament 40.More particularly, a portion of the light passing through lens 42 isdeflected downward by a beam splitter 90 onto detector 48. However,since the distance from the center point 92 of beam splitter 90 todetector 48 is the same as the distance from center point 92 to detector68, the light which reaches detector 48 must pass the same distance asthe light that 10 reaches detector 68. Accordingly, the apparatusschematically illustrated in FIG. 4 is a further improvement over theapparatus illustrated in FIG. 3 in that light bulb 38 can be changed andan accurate measurement of toner concentration obtained irrespective ofthe position of filament 40.

Those skilled in the art will recognize that the specific embodimentsdescribed herein may be altered and modified without departing from thetrue spirit and scope of the invention as defined in the appendedclaims.

What is claimed is:

1. In a printing apparatus for rendering visible a latent electrostaticimage carried on a recording medium by applying to the medium at atoning station a working toner solution comprising a suspension of tonerparticles in a volatile carrier, improved means for maintaining apredetermined concentration of toner particles in the working tonersolution comprising, in combination:

a reservoir of toner particles;

measuring means for continuously measuring the concentration of tonerparticles in the working toner solution, said measuring means includinga light source,

a transparent chamber spaced apart from the light source to receive acontinuously flowing stream of working toner solution,

first light detection means sensitive to light in the near infraredregion of the light spectrum for measuring light intensity from thelight source passing through the chamber, and means for transmittinglight from the light source through the transparent chamber to the firstlight detection means along a first path having a first predeterminedlength;

second light detection means for detecting the intensity of light passedto said chamber from said light source;

means for providing current to the light source;

means for adjusting the current to the light source in response to saidsecond light detection means to maintain a predetermined constant lightintensity from the light source;

transfer means for transferring toner particles from the reservoir tothe working toner solution at a predetermined rate; and

operationmeans responsive to the measuring means for operating thetransfer means when the toner particle concentration decreases below afirst predetermined value and for terminating the operation of thetransfer means when the toner particle concentration increases to thefirst predetermined value.

2. In a printing apparatus for rendering visible a latent electrostaticimage carried on a recording medium by applying to the medium at atoning station a working toner solution comprising a suspension of tonerparticles in a volatile carrier, improved means for maintaining apredetermined concentration of toner particles in the working tonersolution comprising in combination:

a reservoir of toner particles;

measuring means for continuously measuring the concentration of tonerparticles in the working toner solution, said measuring means comprisinga light source, a transparent chamber holding a portion of the workingtoner solution, first light detection means having a positive terminaland a negative terminal for generating a first current signal flowingfrom the negative terminal to the positive terminal in response tolight, first positioning means for positioning the first light detectionmeans to receive light from the light source that is passed through thechamber, second light detection means having a positive terminal and anegative terminal for generating a second current signal flowing fromthe negative terminal to the positive terminal in response to light, andsecond positioning means for positioning the second light detectionmeans to receive light from the light source before the light has passedthrough the working toner solution;

transfer means for transferring toner particles from the reservoir tothe working toner solution at a predetermined rate; and

operating means responsive to the measuring means for operating thetransfer means when the toner particle concentration decreases below afirst predetermined value and for terminating the operation of thetransfer means when the toner particle concentration increases to thefirst predetermined value, said operating means comprising amplifiermeans having a first output and a second output for amplifying andinverting the first current signal with respect to the second currentsignal to produce a first amplified signal having a first voltage on thefirst output corresponding to the first current signal and to produce asecond amplified signal having a second voltage on the second outputcorresponding to the second current signal so that the ratio of thefirst voltage to the second voltage remains constant irrespective of thechanges in the intensity of the light produced by the light source, saidamplifier means comprising a first operational amplifier having aninverting input, a noninverting input and an output comprising saidfirst output; a

means for operatively connecting the positive termi- 1 nal of the firstlight detection means to the noninverting input of the first operationalamplifier and for operatively connecting the negative terminal of thefirst light detection means to the inverting input of the firstoperational amplifier;

a second operational amplifier having an inverting input, a noninvertinginput and an output comprising said second output; and

means for operatively connecting the positive terminal of the secondlight detecting means to the inverting input of the second operationalamplifier and for operatively connecting the negative tenninal of thesecond light detecting means to the noninverting input of the secondoperational amplifier.

3. In a printing apparatus for rendering visible a latent electrostaticimage carried on a recording medium by applying to the medium at atoning station a working toner solution comprising a suspension of tonerparticles in a volatile carrier, improved means for maintaining apredetermined concentration of toner particles in the working tonersolution comprising in combination:

a reservoir of toner particles;

measuring means for continuously measuring the concentration of tonerparticles in the Working toner solution, said measuring means comprisinga light source, a transparent chamber holding a portion of the workingtoner solution, first light detection means having a positive terminaland a negative terminal for generating a first current signal flowingfrom the negative terminal to the positive terminal in response tolight, first positioning means for positioning the first light detectionmeans to receive light from the light source that is passed through thechamber, second light detection means having a positive terminal and anegative terminal for generating a second current signal flowing fromthe negative terminal to the positive terminal in response to light, andsecond positioning means for positioning the second light detectionmeans to receive light from the light source before the light has passedthrough the working toner solution;

transfer means for transferring toner particles from the reservoir tothe working toner solution at a predetermined rate; and 7 operatingmeans responsive to the measuring means for operating the transfer meanswhen the toner particle concentration decreases below a firstpredetermined value and for terminating the operation of the transfermeans when the toner particle concentration increases to the firstpredetermined value, said operating means comprising amplifier meanshaving a first output and a second output for amplifying and invertingthe first current signal with respect to the second current signal toproduce a first amplified signal having a first voltage on the firstoutputcorresponding to the first current signal and to produce a secondamplified signal having a second voltage on the second outputcorresponding to the second current signal so that the ratio of thefirst voltage to the second voltage remains constant irrespective of thechanges in the intensity of the light produced by the light source,

first analog means for algebraically adding and sealing the firstamplified signal and the second amplified signal to produce a firstswitching voltage which varies with the toner particle concentration andwhich has a first predetermined voltage value corresponding to saidfirst predetermined value of the toner particle concentration;

signal generating means for operating the transfer means; and

first switching means for enabling the signal generating means inresponse to a first switching voltage less than the first predeterminedvoltage value and for disabling the signal generating in response to afirst switching voltage greater than the first predetermined voltagevalue..

4. An apparatus according to claim 3 wherein the operating means furthercomprises:

second analog means for algebraically adding and scaling the firstamplified signal and the second amplified signal'to produce a secondswitching signal having a voltage which varies with the toner particleconcentration and which has a second predetermined voltage valuecorresponding to a second predetermined value of the toner particleconcentration less than the first predetermined value of toner particleconcentration; and

second switching means for altering the signal generating means tooperate the transfer means at an increased rate in response to a secondpredetermined voltage less than the second predetermined voltage value.I

5. An apparatus according to claim 4 wherein the first analog meanscomprises a first resistor connected to h fi t Output and a Secondresistor Connected to first switching means comprises a transistorconnected between the first analog means and the control gate.

the Second Output 8. An apparatus according to claim 7 wherein the 6. Anapparatus according to claim 4 wherein the second switching meanscomprises means for IHCI'EQS' signal generating means Comprises a pulsegenerator 5 ing the predetermined repetition rate of the pulses producedby the pulse generator. having a control gate for producing pulses at apredetermined repetition rate.

7. An apparatus according to claim 6 wherein the

1. In a printing apparatus for rendering visible a latent electrostaticimage carried on a recording medium by applying to the medium at atoning station a working toner solution comprising a suspension of tonerparticles in a volatile carrier, improved means for maintaining apredetermined concentration of toner particles in the working tonersolution comprising, in combination: a reservoir of toner particles;measuring means for continuously measuring the concentration of tonerparticles in the working toner solution, said measuring means includinga light source, a transparent chamber spaced apart from the light sourceto receive a continuously flowing stream of working toner solution,first light detection means sensitive to light in the near infraredregion of the light spectrum for measuring light intensity from thelight source passing through the chamber, and means for transmittinglight from the light source through the transparent chamber to the firstlight detection means along a first path having a first predeterminedlength; second light detection means for detecting the intensity oflight passed to said chamber from said light source; means for providingcurrent to the light source; means for adjusting the current to thelight source in response to said second light detection means tomaintain a predetermined constant light intensity from the light source;transfer means for transferring toner particles from the reservoir tothe working toner solution at a predetermined rate; and operation meansresponsive to the measuring means for operating the transfer means whenthe toner particle concentratiOn decreases below a first predeterminedvalue and for terminating the operation of the transfer means when thetoner particle concentration increases to the first predetermined value.2. In a printing apparatus for rendering visible a latent electrostaticimage carried on a recording medium by applying to the medium at atoning station a working toner solution comprising a suspension of tonerparticles in a volatile carrier, improved means for maintaining apredetermined concentration of toner particles in the working tonersolution comprising in combination: a reservoir of toner particles;measuring means for continuously measuring the concentration of tonerparticles in the working toner solution, said measuring means comprisinga light source, a transparent chamber holding a portion of the workingtoner solution, first light detection means having a positive terminaland a negative terminal for generating a first current signal flowingfrom the negative terminal to the positive terminal in response tolight, first positioning means for positioning the first light detectionmeans to receive light from the light source that is passed through thechamber, second light detection means having a positive terminal and anegative terminal for generating a second current signal flowing fromthe negative terminal to the positive terminal in response to light, andsecond positioning means for positioning the second light detectionmeans to receive light from the light source before the light has passedthrough the working toner solution; transfer means for transferringtoner particles from the reservoir to the working toner solution at apredetermined rate; and operating means responsive to the measuringmeans for operating the transfer means when the toner particleconcentration decreases below a first predetermined value and forterminating the operation of the transfer means when the toner particleconcentration increases to the first predetermined value, said operatingmeans comprising amplifier means having a first output and a secondoutput for amplifying and inverting the first current signal withrespect to the second current signal to produce a first amplified signalhaving a first voltage on the first output corresponding to the firstcurrent signal and to produce a second amplified signal having a secondvoltage on the second output corresponding to the second current signalso that the ratio of the first voltage to the second voltage remainsconstant irrespective of the changes in the intensity of the lightproduced by the light source, said amplifier means comprising a firstoperational amplifier having an inverting input, a noninverting inputand an output comprising said first output; means for operativelyconnecting the positive terminal of the first light detection means tothe noninverting input of the first operational amplifier and foroperatively connecting the negative terminal of the first lightdetection means to the inverting input of the first operationalamplifier; a second operational amplifier having an inverting input, anoninverting input and an output comprising said second output; andmeans for operatively connecting the positive terminal of the secondlight detecting means to the inverting input of the second operationalamplifier and for operatively connecting the negative terminal of thesecond light detecting means to the noninverting input of the secondoperational amplifier.
 3. In a printing apparatus for rendering visiblea latent electrostatic image carried on a recording medium by applyingto the medium at a toning station a working toner solution comprising asuspension of toner particles in a volatile carrier, improved means formaintaining a predetermined concentration of toner particles in theworking toner solution comprising in combination: a reservoir of tonerparticles; measuring means for continuously measuring the concentrationof toner particles in the working toner solution, said measuring meanscomprising a light source, a transparent chamber holding a portion ofthe working toner solution, first light detection means having apositive terminal and a negative terminal for generating a first currentsignal flowing from the negative terminal to the positive terminal inresponse to light, first positioning means for positioning the firstlight detection means to receive light from the light source that ispassed through the chamber, second light detection means having apositive terminal and a negative terminal for generating a secondcurrent signal flowing from the negative terminal to the positiveterminal in response to light, and second positioning means forpositioning the second light detection means to receive light from thelight source before the light has passed through the working tonersolution; transfer means for transferring toner particles from thereservoir to the working toner solution at a predetermined rate; andoperating means responsive to the measuring means for operating thetransfer means when the toner particle concentration decreases below afirst predetermined value and for terminating the operation of thetransfer means when the toner particle concentration increases to thefirst predetermined value, said operating means comprising amplifiermeans having a first output and a second output for amplifying andinverting the first current signal with respect to the second currentsignal to produce a first amplified signal having a first voltage on thefirst output corresponding to the first current signal and to produce asecond amplified signal having a second voltage on the second outputcorresponding to the second current signal so that the ratio of thefirst voltage to the second voltage remains constant irrespective of thechanges in the intensity of the light produced by the light source,first analog means for algebraically adding and scaling the firstamplified signal and the second amplified signal to produce a firstswitching voltage which varies with the toner particle concentration andwhich has a first predetermined voltage value corresponding to saidfirst predetermined value of the toner particle concentration; signalgenerating means for operating the transfer means; and first switchingmeans for enabling the signal generating means in response to a firstswitching voltage less than the first predetermined voltage value andfor disabling the signal generating in response to a first switchingvoltage greater than the first predetermined voltage value.
 4. Anapparatus according to claim 3 wherein the operating means furthercomprises: second analog means for algebraically adding and scaling thefirst amplified signal and the second amplified signal to produce asecond switching signal having a voltage which varies with the tonerparticle concentration and which has a second predetermined voltagevalue corresponding to a second predetermined value of the tonerparticle concentration less than the first predetermined value of tonerparticle concentration; and second switching means for altering thesignal generating means to operate the transfer means at an increasedrate in response to a second predetermined voltage less than the secondpredetermined voltage value.
 5. An apparatus according to claim 4wherein the first analog means comprises a first resistor connected tothe first output and a second resistor connected to the second output.6. An apparatus according to claim 4 wherein the signal generating meanscomprises a pulse generator having a control gate for producing pulsesat a predetermined repetition rate.
 7. An apparatus according to claim 6wherein the first switching means comprises a transistor connectedbetween the first analog means and the control gate.
 8. An apparatusaccording to claim 7 wherein the second switching means comprises meansfor increasing the predetermined repetition rate of the pulses producedby the pulse generator.